U.S. patent number 5,938,952 [Application Number 08/787,459] was granted by the patent office on 1999-08-17 for laser-driven microwelding apparatus and process.
This patent grant is currently assigned to Equilasers, Inc.. Invention is credited to Chang-Ming Lin, Richard C. Sam.
United States Patent |
5,938,952 |
Lin , et al. |
August 17, 1999 |
Laser-driven microwelding apparatus and process
Abstract
A microelectronic packaging and interconnection apparatus and
method in which dual ball bonds, metal bumps, or TAB bonds are
formed by pulsed laser irradiation, but not conventional heating,
compression or acoustic (e.g., ultrasonic) methods. A miniature
loop forming tool is used to form a metallic wire loop, one end of
which is welded to an IC bond pad while the other end of which is
welded to an IC package bonding finger, thus forming ball bonds at
both ends of the interconnection.
Inventors: |
Lin; Chang-Ming (San Jose,
CA), Sam; Richard C. (Cupertino, CA) |
Assignee: |
Equilasers, Inc. (Santa Clara,
CA)
|
Family
ID: |
25141544 |
Appl.
No.: |
08/787,459 |
Filed: |
January 22, 1997 |
Current U.S.
Class: |
219/121.64;
219/121.63 |
Current CPC
Class: |
H01L
24/85 (20130101); H01L 24/78 (20130101); H01L
24/86 (20130101); H01L 24/48 (20130101); H01L
24/79 (20130101); B23K 26/032 (20130101); H01L
24/50 (20130101); H01L 2924/01015 (20130101); H01L
2224/4518 (20130101); H01L 2224/786 (20130101); H01L
2224/45015 (20130101); H01L 2224/73265 (20130101); H01L
2224/4845 (20130101); H01L 2224/48465 (20130101); H01L
2224/85205 (20130101); H01L 2924/15165 (20130101); H01L
2924/1517 (20130101); H01L 2924/0106 (20130101); H01L
2924/01013 (20130101); H01L 2924/14 (20130101); H01L
2224/85214 (20130101); H01L 2224/45164 (20130101); H01L
2224/48724 (20130101); H01L 2924/01074 (20130101); H01L
24/45 (20130101); H01L 2924/01078 (20130101); H01L
2924/1532 (20130101); H01L 2224/851 (20130101); H01L
2224/48091 (20130101); H01L 2224/85203 (20130101); H01L
2924/01079 (20130101); H01L 2924/15153 (20130101); H01L
2224/48227 (20130101); H01L 2224/48699 (20130101); H01L
2224/78301 (20130101); H01L 2924/01029 (20130101); H01L
2924/01027 (20130101); H01L 2924/01042 (20130101); H01L
2924/10253 (20130101); H01L 2224/48824 (20130101); H01L
2924/15312 (20130101); H01L 2224/8501 (20130101); H01L
2224/45144 (20130101); H01L 2224/48624 (20130101); H01L
2924/01006 (20130101); H01L 2224/85181 (20130101); H01L
2924/01039 (20130101); H01L 2224/45147 (20130101); H01L
2224/48464 (20130101); H01L 2924/01014 (20130101); H01L
2224/92247 (20130101); H01L 2924/12042 (20130101); H01L
2924/12043 (20130101); H01L 2924/01005 (20130101); H01L
2924/01051 (20130101); H01L 2224/05624 (20130101); H01L
2924/014 (20130101); H01L 2924/01327 (20130101); H01L
2224/45124 (20130101); H01L 2224/45144 (20130101); H01L
2924/00014 (20130101); H01L 2224/45147 (20130101); H01L
2924/00014 (20130101); H01L 2224/48091 (20130101); H01L
2924/00014 (20130101); H01L 2224/85181 (20130101); H01L
2224/48465 (20130101); H01L 2224/48465 (20130101); H01L
2224/48227 (20130101); H01L 2224/48465 (20130101); H01L
2224/48227 (20130101); H01L 2924/00 (20130101); H01L
2224/85205 (20130101); H01L 2224/45147 (20130101); H01L
2924/00 (20130101); H01L 2224/85205 (20130101); H01L
2224/45144 (20130101); H01L 2924/00 (20130101); H01L
2224/85205 (20130101); H01L 2224/48465 (20130101); H01L
2924/00 (20130101); H01L 2224/48465 (20130101); H01L
2224/48091 (20130101); H01L 2924/00 (20130101); H01L
2224/48699 (20130101); H01L 2924/00 (20130101); H01L
2924/10253 (20130101); H01L 2924/00 (20130101); H01L
2224/45015 (20130101); H01L 2924/00 (20130101); H01L
2224/45144 (20130101); H01L 2924/00015 (20130101); H01L
2224/45124 (20130101); H01L 2924/00015 (20130101); H01L
2224/45015 (20130101); H01L 2924/20753 (20130101); H01L
2224/48824 (20130101); H01L 2924/00 (20130101); H01L
2224/48624 (20130101); H01L 2924/00 (20130101); H01L
2224/48724 (20130101); H01L 2924/00 (20130101); H01L
2224/45015 (20130101); H01L 2924/00014 (20130101); H01L
2924/20753 (20130101); H01L 2224/45147 (20130101); H01L
2924/00015 (20130101); H01L 2224/45164 (20130101); H01L
2924/00014 (20130101); H01L 2924/12042 (20130101); H01L
2924/00 (20130101); H01L 2924/12043 (20130101); H01L
2924/00 (20130101); H01L 2224/85203 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
B23K
26/02 (20060101); B23K 26/03 (20060101); B23K
026/00 () |
Field of
Search: |
;219/121.63,121.64,121.76,121.77,121.83,121.85,121.62
;228/4.5,6.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
3831394 |
|
Mar 1990 |
|
DE |
|
57-175093 |
|
Oct 1982 |
|
JP |
|
61-039536 |
|
Feb 1986 |
|
JP |
|
63-79331 |
|
Apr 1988 |
|
JP |
|
1-199443 |
|
Aug 1989 |
|
JP |
|
5-102232 |
|
Apr 1993 |
|
JP |
|
5-347308 |
|
Dec 1993 |
|
JP |
|
Primary Examiner: Evans; Geoffrey S.
Attorney, Agent or Firm: Coudert Brothers
Claims
What is claimed is:
1. A method of interconnecting through dual-ball bonding a first
and a second bonding pads spatially separated from each other,
comprising the steps of:
providing a length of an electrically conductive material;
irradiating with a first laser pulse a first end portion of said
length of said conductive material causing said first end portion
to momentarily change state and deform into an essentially
spherical shape;
placing said deformed first end portion into contact with said
first bonding pad;
irradiating with a second laser pulse said deformed first end
portion and said first bonding pad to create a first microweld
ohmically connecting said deformed first end portion to said first
bonding pad;
displacing said length of said conductive material away from said
first bonding pad to form a wire loop of said conductive material
having a first end portion welded to said first bonding pad and a
second end portion integrally connected to said length of said
conductive material;
irradiating said length of said conductive material with a third
laser pulse at said second end portion of said wire loop to sever
said wire loop from said length of said conductive material and to
deform said second end portion of said wire loop into an
essentially spherical shape;
placing said deformed second end portion of said wire loop into
contact with said second bonding pad; and
irradiating with a fourth laser pulse said deformed second end
portion of said wire loop and said second bonding pad to create a
second microweld ohmically connecting said deformed second end
portion of said wire loop to said second bonding pad, thereby
causing said wire loop to interconnect said first and said second
bonding pads.
2. A method as recited in claim 1 wherein, prior to irradiating
with said first laser pulse said first end portion of said length
of said conductive material, said first end portion is mechanically
flattened to increase the area exposed to said first laser
pulse.
3. A method as recited in claim 2 wherein, prior to irradiating
with said third laser pulse said length of said conductive material
at said second end portion of said wire loop, said second end
portion of said wire loop is mechanically flattened to increase the
area exposed to said third laser pulse.
4. A method as recited in claim 3 wherein said electrically
conductive material is selected from the group consisting of
aluminum, gold, copper and palladium.
5. A method as recited in claim 1 wherein irradiating said length
of said conductive material at said second end portion of said wire
loop creates a new first end portion of the remaining length of
said conductive material, said new first end portion deformed into
an essentially spherical shape.
6. A method as recited in claim 5 wherein, prior to irradiating
with said first laser pulse said first end portion of said length
of said conductive material, said first end portion is mechanically
flattened to increase the area exposed to said first laser
pulse.
7. A method as recited in claim 6 wherein, prior to irradiating
with said third laser pulse said length of said conductive material
at said second end portion of said wire loop, said second end
portion of said wire loop is mechanically flattened to increase the
area exposed to said third laser pulse.
8. A method as recited in claim 7 wherein said electrically
conductive material is selected from the group consisting of
aluminum, gold, copper and palladium.
9. A method as recited in claim 1 wherein said electrically
conductive material is selected from the group consisting of
aluminum, gold, copper and palladium.
10. A method for bonding a tape-automated bonding (TAB) tape having
a plurality of leads to an electronic element having a plurality of
bonding areas, comprising the steps of:
irradiating the end of each of said leads with a first laser beam
to deform each of said ends into an essentially spherical
shape;
contacting each said deformed lead end with one of said bonding
areas; and
irradiating with a second laser beam each said deformed lead end
and said bonding area in contact therewith to create a microweld
between said deformed lead end and said bonding area.
11. An apparatus for forming a bump made of a portion of a wire on
a surface, comprising:
a first laser source for generating a first laser beam;
a second laser source for generating a second laser beam;
means for coupling said first laser beam and said second laser
beam;
means for projecting said laser beams to said surface;
means for deforming an end portion of said wire;
means for moving one or more of said laser beams, said deforming
means, said wire and said surface; and
means for monitoring and controlling said laser beams.
12. An apparatus as recited in claim 11, further comprising:
means for separating said wire from said surface.
13. A method for forming an essentially hemispherical, electrically
conductive bump on an electrically conductive surface, comprising
the steps of:
providing a length of an electrically conductive material;
irradiating with a first laser pulse an end portion of said length
of said conductive material to deform said end portion into an
essentially spherical shape;
placing said deformed end portion in contact with a portion of said
conductive surface;
irradiating with a second laser pulse said deformed end portion and
said portion of said conductive surface to form said bump, said
bump welded to said conductive surface and connected to said length
of said conductive material; and
irradiating with a third laser pulse the top portion of said bump
to separate said bump from said length of said conductive
material.
14. A method as recited in claim 13 wherein said electrically
conductive material is selected from the group consisting of
aluminum, gold, copper, palladium, molybdenum and solder.
15. An apparatus for bonding a wire to both a first bonding pad and
a second bonding pad through dual-ball bonding, said first and said
second bonding pads spatially separated from each other,
comprising:
a first laser source for generating a first laser beam;
a second laser source for generating a second laser beam;
means for coupling said first laser beam and said second laser
beam;
means for projecting said laser beams to the surface of said first
bonding pad and the surface of said second bonding pad;
means for forming a wire loop out of a portion of said wire;
means for clamping portions of said wire;
means for moving one or more of said laser beams, said clamping
means, said means for forming said wire loop, said wire and said
wire loop; and
means for monitoring and controlling said laser beams.
16. An apparatus as recited in claim 15, further comprising:
means for moving one or more of said first bonding pad and said
second bonding pad.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to microelectronic packaging and
interconnection, and more particularly, to apparatuses and methods
in which dual ball bonds, metal bumps or TAB bonds are formed by
laser, but not conventional heating, compression or acoustic,
means.
2. Brief Description of the Prior Art
Wire bonding has been one of the most important techniques for
making microelectronic interconnections. Two types of wire bonding
methods are widely used in IC package assembly operations: the gold
ball bonding method and the wedge bonding method. The gold ball
bonding method, despite being so named, involves the formation of a
wire loop with a ball bond at one end but a wedge bond at the other
end. This bonding method is only suitable for gold or slightly
doped gold wires. On the other hand, the wedge bonding method
delivers wedge bonds at both ends of the wire loop and is mostly
used for aluminum wire bonding, although gold wires are
occasionally used. Generally speaking, between these two methods,
the gold ball bonding method delivers better throughput and shorter
wire loop length while the aluminum wedge bonding method gives
relatively finer bonding pitch.
In either of the above wire bonding methods, mechanical means,
e.g., compression or ultrasound, or the combinations thereof, are
used to form the ball bonds and the wedge bonds. In addition,
thermal energy must be provided to the substrate during a gold ball
bonding process. Thus, depending on the specific mechanical means
involved, there are two well-established gold ball bonding
techniques: thermocompression bonding and thermosonic bonding. In
both techniques, thermal energy is supplied by either conventional
(e.g., resistive) heating means or laser heating to keep the
substrate at an elevated temperature during the formation of the
bonds to ensure adequate bonding strength.
For example, U.S. Pat. No. 4,529,115, issued to T. Renshaw et al.
and entitled "Thermally Assisted Ultrasonic Welding Apparatus and
Process," teaches a conventional thermally assisted ultrasonic
bonding device including an electrical resistance coil wrapped
around the bonding tip. A limitation of this device is that the
thermal mass being heated is very large compared to the specific
bonding area. Moreover, an elevated temperature of the thermal mass
may not only pose a safety hazard to the operator but also be
detrimental to the operability of the adjacent or integral heat
sensitive components.
As another example, U.S. Pat. No. 4,534,811, issued to N. Aisnlie
et al. and entitled "Apparatus for Thermo Bonding Surfaces,"
describes a conventional laser-assisted ultrasonic bonding device
including a laser and a hollow ultrasonic bonding tip. The
combination of heat supplied by the laser and the sonic energy
supplied by the bonding tip offers the ability to dynamically
provide bonding energy in a short pulse to a limited bonding area.
However, the laser component taught in this reference is expensive,
complex and incompatible with many modern bonding processes.
Excessive ultrasonic forces may also be detrimental to the
integrity of electronic components embedded beneath the bonding
area. For example, bond pad "cratering" may be caused by either
excessive ultrasonic forces or compression.
As further examples, U.S. Pat. No. 4,718,591, issued to W. Hill and
entitled "Wire Bonder With Open Center of Motion," discloses an
ultrasonic bonding machine having an open center mounting that
permits optimum, substantially linear motion of a bonding tip,
whereas U.S. Pat. No. 4,598,853, issued to W. Hill and entitled
"Open Center Flexural Pivot Wire Bonding Head," discloses a
flexural pivot structure useful in an apparatus for bonding thin
wire leads in microelectronic circuits.
Attempts have also been made to use thermal energy alone to bond
electrical or electronic components. An example is U.S. Pat. No.
3,718,968, issued to Sims et al. and entitled "Method for
Connecting a Wire to a Component," which discloses a bonding method
including the following steps: deforming the end of a wire by
heating it above its melting point with a laser beam and permitting
it to solidify into a sphere; placing the sphere in contact with
the component; and heating the sphere and the bonding area of the
component by a laser beam above their respective melting points to
provide a fusion weld between the wire and the component.
Essentially, this reference teaches that, to avoid vaporization of
the end of the wire before the bonding area is melted, the sphere
must be made as large as necessary to provide sufficient reduction
in its area-to-volume ratio, and that the duration of laser heating
must be long enough to raise the temperatures of both the sphere
and the component bonding area above their respective melting
points but not above the vaporization point of the sphere. These
requirements, however, are over-simplified ones, and are at best a
necessary, but not sufficient, condition for successful
laser-driven bonding.
Having thus described prior-art technology disclosed in several
issued patents, a schematic representation of a conventional,
"generic" gold ball bonding apparatus and method is depicted in
FIGS. 1A-1E. In FIG. 1A, a wire made of gold or specially doped
gold is passed through a bonding tip or "capillary" 12, which is an
essential part of the conventional ball bonding apparatus 14. The
capillary 12 itself may or may not be heated. An electric spark 16
is used to melt the end of the wire 10 to form a ball 18; this is
known as the "electric flame-off" or "EFO" step. The newly formed
gold ball 18 is brought to the close proximity of a bond pad 20 on
an IC chip (i.e., the substrate) 22, which is typically heated to
300-400.degree. C. A somewhat lower substrate temperature may be
used if sonic energy is applied in conjunction with substrate
heating.
Next, as depicted in FIG. 1B, the capillary 12 presses the ball
against the bond pad 20 during a short ultrasonic pulse to form a
gold ball bond 24 in the form of a "nail head," the bottom of which
is bonded to the bond pad 20. This is commonly known as the "ball
bonding" step.
In FIG. 1C, the capillary 12 is shown lifted from the substrate 22
to allow the newly formed gold ball bond 24 to unreel the wire 10
through the capillary 12.
In FIG. 1D, the capillary 12 is shown moved away from the chip 22
and repositioned above a bonding finger (or lead frame) 26 of an IC
package 28; as a result, a wire loop 30 is formed.
Finally, as depicted in FIG. 1E, a wedge bond 32 is formed by
shearing the gold wire on the bonding finger 26 of the IC package
28; that is, the capillary 12 presses the gold wire against the
bonding finger 26 and moves essentially horizontally away from the
bonding finger 26. The capillary 12 is then raised while a small
length of wire is unreeled. A prior-art electromagnetic wire clamp
(not shown) is actuated to break the wire at the neck of the wedge
32, leaving a flattened tail adhering to the bonding finger 26.
Thus, the IC bond pad 20 and a bonding finger 26 are electrically
connected through the gold wire loop 30, one end of which is
connected to the IC bond pad 20 via a ball bond 24 while the other
end is connected to the bonding finger 26 via a wedge bond 32. This
wire bonding process can be repeated for the connection of the
remaining bond pads of the IC chip to the corresponding bonding
fingers of the IC package, creating a completed and functional IC
device.
The aforementioned ball bonding apparatus and method works fairly
well for wires made of soft metals such as gold. However, many
problems do occur. First, to allow the wire to pass through its
center, the conventional capillary is usually quite bulky, setting
a limit on the finest bonding pitch achievable with the gold ball
bonding technique. Second, the EFO step often damages the capillary
and significantly shortens the useful life of the capillary. Third,
to ensure good contact between the wire and the bonding surface at
a wedge bond, the capillary generally has to make a low-entry-angle
motion relative to the bonding surface. This low angle necessarily
entails a longer wire loop and longer "interpad" distance between
the bond pad of the chip and the bond pad or bonding finger of the
package than if the wire loop could come down at the package
bonding plane at a greater entry angle, i.e., closer to 90.degree..
Fourth, because the IC chips are subjected to substrate heating and
mechanical stress caused by compression or ultrasonic forces,
various forms of IC chip damages may occur, resulting in a lower
overall yield. If wires made of metals possessing greater hardness
and higher melting points, such as copper or palladium, were used,
the overall yield would be even lower, because higher substrate
temperature and more mechanical energy would be required to achieve
bonding. In fact, such IC damage and yield problems have
practically prevented fine copper or palladium wires from being
widely used in wire-bonded microelectronic packages, even though
these metals are both electrically better (e.g. greater
electromigration resistance) and mechanically stronger than gold.
This limitation of conventional wire bonding is well documented in
the literature.
Note that, compared to the above ball bonding method, the
conventional wedge bonding method requires an even greater interpad
distance because of the low entry angles dictated by the wedge
bonds at both the chip bond pad and the package bond pad.
The aforementioned conventional gold ball bonding method can be
modified to produce gold bumps on an IC chip. Such gold bumps may
be used in attaching the IC chip directly to a circuit board or an
IC package (either a ceramic package, plastic package, or a
flexible circuit board) via the flip-chip method, or in other
advanced packaging assembly applications such as MCM (multichip
module), MEMS (mechanical-electronic microsensor), and hard disk
assembly.
FIGS. 2A-2C depict this conventional "metal stub" method for gold
bump formation. In FIG. 2A, a gold ball 18 is formed by the
aforementioned EFO process. The newly formed ball 18 is then
brought in contact with a bond pad 20 of an IC chip 22; see FIG.
2B. A ball bond 24 in the form of a nail head is produced on the
bond pad by the aforementioned ball bonding step. As shown in FIG.
2C, the capillary 12 is raised to unreel a small length of wire 10.
A prior-art wire clamp 16 is then electromagnetically actuated to
break the wire 10 right above the nail head 24, leaving a gold bump
having a short and sharp tail 34. The above steps depicted in FIGS.
2A-2C may be repeated to generate gold bumps on all of the bond
pads of the IC chip so that the bond pads can be directly attached
to a circuit board or other connecting structures via the
bumps.
Similar to the above formation of ball bonds between an IC chip and
gold wires in the conventional ball bonding method, the substrate
in the aforesaid prior-art gold bumping method is generally kept at
an elevated temperature and subjected to vicious mechanical
compression and shear to ensure acceptable bond strength. Thus, the
IC damage and yield problems cited above for the conventional ball
bonding method persist in this conventional gold bumping method as
well. Furthermore, because the finished gold bumps have
uncontrolled sharp tails due to the nature of metal fracture,
additional planarization processes are generally required to
flatten these sharp tails before the chip can be attached to a
circuit board or another structure via, e.g., the flip-chip
process. Finally, the average height of finished gold bumps
produced by this conventional gold bumping process is usually
several mils tall, making this process unsuitable for today's
miniaturized IC packaging assemblies.
Another microelectronic packaging technique that has showed great
promises is the tape-automated-bonding (commonly dubbed TAB)
assembly process. TAB was originally developed as a highly
automatic technique for packaging large volume, low I/O devices,
but has since been applied to high I/O devices (more than 300 I/O
connections) as well. Typically, the TAB process involves the use
of thermal compression bonding to bond silicon chips to metal
(e.g., copper) strips deposited and patterned on a polymer (e.g.,
polyimide or Mylar) tape. Thus, TAB technology is generally
considered as both a chip connection method and a first-level
packaging technique.
A typical example of a TAB tape 40 currently used in package
industry is shown in FIG. 3A. This TAB tape 40 consists of
laminated and patterned copper leads 42 glued to prepunched
high-temperature polymer film 44. The copper leads 42 may also be
coated with an electroplated solder layer to optimize the TAB
bonding operation and provide better corrosion resistance. This
solder electroplating process requires that all copper leads be
shorted to electrode contact points. This adversely affects the
usable area on the tape, and additionally, may require a special
cutting or punching operation to delete common connections where it
is desirable to test Inner Lead Bonding (ILB) bonded devices prior
to Outer Lead Bonding (OLB).
In principle, to bond IC chips to a TAB tape, bump contacts can be
made on either the IC chip or the TAB tape. However, formation of
bumped TAB tapes is rather complicated and expensive. This makes
the use of bumped chip the method of choice for TAB bonding.
An example of a conventional bumped chip TAB bonding process is
illustrated in FIG. 3B. Through a series of time-consuming and
labor-intensive processes, including thin film deposition,
photoresist coating, photolithography alignment and exposure,
development of photoresist, solder electroplating, post-plating
etching, etc. Solder bumps 46 are generated on the IC chips 48
while the chips 48 are still in wafer form. Subsequently, the
bumped wafer is examined and sawed to liberate each individual IC
chip 48. These IC chips 48 are then transferred onto a sticky or
wax layer 50 attached to a polymer tape carrier 52. These bumped IC
chips 48 are affixed to the copper leads 42 of the TAB tape 40 as
follows. First, the tips of the copper leads 42 are visually
aligned to the corresponding solder bump contacts 46 of the IC chip
48 via an optical alignment system. Then, the copper leads 42 are
compressed down against the solder bump contacts 46 and heated by a
resistive heating block 54, with or without ultrasound, forming
bonding between each copper lead 42 and its corresponding solder
bump contact 46. Alternatively, as shown in FIG. 3C, the resistive
heating block 54 can be replaced by a hollow capillary 56 to
accommodate a low-power laser beam 58 which provides heat needed
for the lead-to-bump bonding.
As an example, U.S. Pat. No. 4,893,742, issued to P. Bullock and
entitled "Ultrasonic Laser Soldering," teaches a "flux-less" TAB
laser soldering apparatus, where an elongated ultrasonically
vibratable capillary is employed to press the wire down against its
pad and to receive and guide an optical fiber through which a beam
of laser energy is fed coaxially through the capillary to heat the
wire end.
In addition to possibly excessive heating and mechanical stress, a
severe limitation of the conventional TAB bonding technique is that
the unbumped areas of the copper leads and the IC chip must
generally be masked with a patterned photoresist layer prior to
solder plating. This requires additional care, process steps, and
process time. Furthermore, either the TAB tape or the IC chip, or
both, are exposed to undesirable wet chemicals during the
photolithographic masking process and the electrochemical plating
process. Thus, the conventional TAB bonding method has been
regarded by many skilled in the art as a messy, complex, and
time-consuming process. Such complexity of the conventional TAB
process also entails a significantly higher operational cost than
wire bonding methods, preventing the TAB technique from being
widely accepted by the cost-conscious microelectronic packaging
industry.
The aforesaid problems, i.e., substrate damage caused by thermal
and mechanical means, reduction in the overall yield, damage to the
capillary, needs for photolithographic masking and electroplating,
etc., will become worse and worse as the size of IC chips moves
towards miniaturization; that is, I/O pads and transistors shrink
in size and increase in density. Hence, there is a need in the art
for a microwelding apparatus and process which allows finer pitch
bonds and bumps without the need for photolithographic masking,
electrochemical plating, or excessive heating, compression or
ultrasonic vibration.
All of the patents mentioned above are hereby incorporated by
reference for purposes of additional disclosure.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
ball bonding apparatus and process suitable for
high-bonding-strength, short-interpad-distance, fine-pitch
microelectronic packaging without the use of substrate or package
heating or conventional mechanical means, e.g., ultrasound,
compression or shearing.
Another object of the present invention is to provide a ball
bonding apparatus and process which allows the use of wires made of
hard or high-melting-point metals, e.g., copper or palladium.
Still another object of the present invention is to provide an
improved metal bumping apparatus and flux-less bumping process for
generating fine metal bumps requiring none of the following:
substrate or package heating, use of ultrasound or compression, and
postbumping planarization.
Yet another object of the present invention is to provide a metal
bumping apparatus and process which enables the formation of bumps
made of metal other than gold, e.g., copper, palladium, molybdenum,
or solder alloys.
A further object of the present invention is to provide a
simplified TAB apparatus and flux-less bumping process for bonding
IC chip bond pads to the TAB tape without the use of any of the
following: photolithographic masking, eletrochemical plating,
acoustic energy, compression stress, and substrate or package
heating.
Still a further object of the present invention is to eliminate
several problems including substrate overheating, bond pad
cratering damage, and bonding tip damage, all of which are commonly
associated with conventional gold ball bonding, metal stub bumping,
and TAB assembly processes.
Yet a further object of the present invention is to increase the
overall yield of microelectronic packaging by providing an improved
laser-driven bonding apparatus and process.
According to one aspect of the present invention, the end of a wire
made of gold, copper, palladium or other materials is melted into a
ball, and the ball is welded to a bond pad of a microelectronic
component (e.g., an IC chip) through the use of pulsed high-energy
laser irradiation. No capillary-type bonding tip, electric
flame-off, substrate heating, or ultrasound or compression is used
in this new bonding process. A miniature loop forming tool is used
to form a wire loop, one end of which is welded to the aforesaid
bond pad and the other end of which is welded to a bond pad of
another microelectronic component (e.g., an IC package), again
through the use of the aforesaid pulsed high-energy laser
irradiation. A ball bond, instead of wedge bond, is formed at each
end of the wire connecting the microelectronic components,
resulting in an interpad distance well-suited for miniaturized
microelectronic packaging. Because the amount of superfluous heat
is greatly reduced in comparison to conventional bonding
techniques, very little thermal damage to the microelectronic
components takes place, nor is there much formation of interfacial
intermetallic compounds at the microwelding area.
According to another aspect of the present invention, a first
pulsed high-energy laser irradiation is used to locally melt and
deform the end of a conductor wire made of gold, copper, palladium,
molybdenum, solder or other materials. A second laser irradiation
is used to turn the deformed wire end into an essentially
hemispherical bump on the surface of a bond pad of a
microelectronic component (e.g., an IC chip) and to form a strong
alloy bond between the bump and the bond pad. A wire clamping tool
or a third laser irradiation is used to break the wire right above
the bump; the use of the laser has the advantage of restoring the
bump into an essentially hemispherical shape without additional
planarization.
In according with yet another aspect of the present invention,
pulsed high-energy laser irradiation is used to form a copper ball
at the tip of each copper lead of a standard TAB tape. The copper
balls and the matching bond pads of a microelectronic component
(e.g., an IC chip) are brought into contact, followed by another
pulsed high-energy laser irradiation to form a strong alloy bond
between each copper ball and its corresponding bond pad. No
lithographic or electroplating processes are involved in this new
TAB bonding process.
An advantage of the present invention is that the new ball bonding
apparatus and process can produce microelectronic packaging with
high-strength, high-reliability gold, copper or palladium ball
bonds at both the chip and the package end, without the need for
sophisticated mechanical means and without suffering substrate
damage or overall yield reduction.
Another advantage of the present invention is that the new ball
bonding apparatus and process can produce shorter wire loops and
allow shorter interpad distances, thus reducing the dimensions of
microelectronic packages and facilitates the use of advanced
packaging techniques in applications such as MCMs and MEMS.
Still another advantage of the present invention is that it is
possible to use a coated magnetic or conductive wire because the
concentrated laser energy can evaporate the polymer coating of such
a wire at the microwelding spot and create a direct contact between
the wire itself and the target substrate.
Another advantage of the present invention is that the new
flux-less metal bumping apparatus and process can produce metal
bumps with well-controlled dimensions suitable for advanced IC
interconnect techniques such as flip chips or the fabrication of
ball/bump grid array (BGA) packages.
Yet another advantage of the present invention is that the new TAB
apparatus and process can produce high-strength TAB assemblies
without costly and messy lithographic masking and electrochemical
plating processes required by conventional TAB processes.
A further advantage of the present invention is that it generally
consumes less thermal energy compared with prior-art bonding and
bumping methods.
Still a further advantage of the present invention is that the
laser microwelding approach is relatively insensitive to bond pad
surface contamination because localized laser power helps clean up
the bond pad surface.
Yet a further advantage of the present invention is that
single-metal bumps can be made to replace solder bumps in BGA
packages and other similar packages, thus not only greatly
simplifying the reflow process dictated by the use of solder, but
also improving the performance at electrical joints.
Another advantage of the present invention is that not only metals
but other inorganic or organic materials may be used in the new
bonding process, e.g., connecting an optical glass fiber to an
optoelectronic package.
These and other objects, features and advantages of the present
invention will no doubt become apparent to those skilled in the art
after having read the following detailed description of the
preferred embodiments which are illustrated in the several figures
of the drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1A-1E are schematic illustrations of a conventional prior-art
wire bonding process;
FIGS. 2A-2C are schematic illustrations of a conventional prior-art
metal bumping process;
FIGS. 3A-3C are schematic illustrations of a conventional prior-art
TAB assembly process;
FIG. 4 is a schematic representation of an optical setup of a
laser-driven microwelding system in accordance with the present
invention;
FIGS. 5A and 5B are perspective views illustrating two loop-forming
tool embodiments in accordance with the present invention;
FIG. 6 is a perspective view illustrating a laser-driven
microwelding system in accordance with the present invention;
FIGS. 7A-7J are schematic illustrations of a laser-driven dual-ball
bonding process in accordance with the present invention;
FIGS. 8A-8G are schematic illustrations of a laser-driven metal
bumping process in accordance with the present invention; and
FIGS. 9A-9C are schematic illustrations of a laser-driven TAB
assembly process in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the present invention may be embodied in many forms, details
of the preferred embodiments are schematically shown in FIGS. 4-9,
with the understanding that the present disclosure is not intended
to limit the invention to the embodiments illustrated.
In one specific embodiment of the present invention, a conventional
wire bonding apparatus, similar to the one depicted in FIG. 1, is
modified to provide an improved laser-driven microwelding apparatus
in which wire bonding between elements, e.g., IC chips, conductor
wires and electronic packages, is driven essentially by laser and
not by substrate heating or extraneous mechanical means, e.g.,
compression, ultrasound or shearing. Alternatively, the new
laser-driven microwelding apparatus may be specifically built in
the manner taught below, with a laser being the exclusive energy
source for wire bonding.
There are three essential subsystems in the new laser-driven
microwelding apparatus, i.e., a coupled laser
source/focusing/monitoring subsystem, a precision displacement
subsystem, and a loop-forming tool. FIG. 4, FIGS. 5A and 5B, FIG.
6, and FIGS. 7A-7J respectively and collectively, illustrate
several aspects of these subsystems as well as how they function in
conjunction with one another.
Referring to FIG. 4, the laser-driven microwelding apparatus
comprises a prior-art wire bonding apparatus known to one skilled
in the art (not shown), and a coupled laser
source/focusing/monitoring subsystem 112 of the present invention.
Also shown in FIG. 4 is a loop-forming tool 114. This subsystem 112
includes a laser source 116 generating a confined pulsed laser beam
118, an optics compartment 120, a second laser source 122
generating a second laser beam 124, an optical fiber 126, and a
beam imaging assembly 128. The coupled laser beam 130 emitted from
both laser sources 116 and 122 is transmitted through a
beamsplitter compartment 132, a monitoring system consisting
essentially of a camera 134 and a monitor 136, and a prior-art
Z-axis linear precision displacement motor (not shown) coupling the
positioning of the laser beam 130 with the X,Y-axis linear
precision displacement of a substrate 140.
Referring again to FIG. 4, the laser source 116 is capable of
generating on demand laser pulses 118, which, in combination with
laser beam 124, are delivered to a bond pad 138 of the substrate
140 via optical fiber 126, laser beam imaging assembly 128 and
beamsplitter compartment 132. With such laser pulses, a conductor
wire made of, e.g., gold, copper or gold, may be melted and
amalgamated with aluminum bond pad 138 at the microwelding spot.
These laser pulses should, on the one hand, deliver sufficient
thermal energy to the bonding area for microwelding purposes and,
on the other hand, maximize the efficient use of thermal energy by
rapidly heating mainly the microwelding spot and minimize the
amount of thermal exposure to the IC substrate 140.
Among commercially available lasers, the pulsed Nd:YAG laser is
emitting at 1.064 .mu.m (micrometers) particularly suitable as the
laser source 116, although other laser sources may also be used.
Two convenient sources for such Nd:YAG lasers are Equilasers, Inc.
(Sunnyvale, Calif.) and Miyachi Co, (Tokyo, Japan). For example,
Equilasers Model EDW-15, having a energy rating of 150 mJ-12 J, is
particularly suitable for the purpose of the present invention.
Depending on the bonding materials, the laser source 116 may
deliver a laser beam with a power density ranging from
approximately 1.times.10.sup.3 to 8.times.10.sup.6 W/cm.sup.2 over
a duration ranging from a few microseconds to a few milliseconds.
For example, when microwelding a copper wire to an aluminum bond
pad, the laser source typically provides laser power density at a
density approximately 4.7.times.10.sup.5 W/cm.sup.2 for
approximately 0.5 millisecond. Thus, laser energy used for ball
bonding involving a 1.3-mil diameter copper wire is approximately
0.30. Joule per pulse. The amalgamated Cu--Al ball bonds thus
generated have a bond strength far exceeding 5.times.10.sup.7
dyne/cm.sup.2, which is much greater than 15 grams per ball bond.
This strength is comparable to the bulk Cu--Al alloy strength, and
is far superior to typical gold--gold or gold-aluminum ball bonds
generated by a typical thermosonic bonding tool, where the bond
strength can be as low as 3 grams. This is mainly because problems
of interfacial adhesion integrity and intermetallic compounds,
which so often plague gold or gold-aluminum ball bonds, is not a
threat in copper-aluminum ball bonds. There is no sign of necking,
i.e., narrowing of the wire near the ball bond, either. In
addition, because of the short duration of the laser pulse,
substrate heating is minimized, resulting in virtually no substrate
damage except for, occasionally, a faint radiant recrystallization
morphology around the ball bond. If necessary, this radiant
morphology can be further mitigated by minor engineering effort,
e.g., keeping the laser power parameters, i.e., laser energy,
welding duration, etc., at their respective optimum values.
The main purpose of the second laser source 122 in FIG. 4 is to
provide an aiming beam 124 so that the high-energy laser beam 118
from the first laser source 116 can be accurately positioned and
focused at the bond pad 138. Another purpose of the second laser
source 122 is that the laser beam 124 emitted therefrom can assist
the positioning of the loop forming tool 114 during the bonding
operation. A laser well-suited for this purpose is a visible (e.g.,
red) laser diode, which is commercially available from companies
such as SDL, Inc. (San Jose, Calif.) and Opto-Power Co. (Tucson,
Ariz.). The optical compartment 120 includes a beam splitter, a
photodiode detector, and a lens system, all of which, collectively,
couple the laser beam 118 from the first laser source 116 with the
laser beam 124 from the second laser source 122. The two input
laser beams may be coupled in a number of ways; for example, they
may be superimposed. The coupled laser beam 130 is then transmitted
through the optical fiber 126 and emitted from the distal end to
the beam imaging assembly 128.
The beam imaging assembly 128 includes a pair of lenses 142 and
144, which image the laser beam 130 from the optical fiber into a
small spot in the conjugate position. The imaging ratio, i.e., the
ratio of the incoming to focused beam sizes, may be varied to yield
different image sizes. For the best result, the imaging ratio for
the present application is preferably in the range of 1:1 to
2:1.
The beamsplitter compartment 132 is attached to the beam imaging
assembly 128 and contains a beamsplitter 146. By properly adjusting
the transmittance and reflectance of the beamsplitter 146, as known
to those skilled in the art, the beamsplitter 146 can be made to
reflect almost 100% of the high-energy laser beam and approximately
50% of the visible aiming beam towards the microwelding spot. The
entry angle of the laser beam with respect to the surface of the
substrate 140 should be so chosen that the beam will not cause a
safety hazard to the operator of the microwelding apparatus.
Approximately half of the visible aiming beam 124 reflected from
the microwelding spot reaches the camera (e.g., a CCD camera) 134,
due to the 50% transmission efficiency of the beamsplitter 146. The
CCD camera 134 captures a view of the substrate 140, on which the
aiming beam 124 is also focused through the beamsplitter 146. This
view may be continuously displayed on the monitor 136 and used for
aligning the laser beam 130 with the microwelding spot.
The beam imaging assembly 128, the beamsplitter compartment 132 and
the CCD camera 134 may conveniently all be packaged in an assembly
135 (shown below in FIG. 6), which can be translated vertically
(i.e., in the Z-direction) with an accuracy of 1 .mu.m by a linear
precision translation stage known to those skilled in the art. The
selection of the bonding sites on the substrate (i.e., in the X and
Y directions) is conveniently controlled by a linear precision
translation stage with an accuracy of 1 .mu.m, also known to those
skilled in the art. The selection of bonding sites can be
accomplished through either the motion of this mechanical assembly
or the motion of the IC chip, or a combination of both. As known to
those skilled in the art, depending on the application, motions in
the three directions can be either coupled together or decoupled as
independent motions.
Having described several portions of the novel laser-driven
microwelding apparatus of the present invention, attention is
directed toward the third, i.e., a loop forming tool, which is used
to form a wire loop linking the IC chip and the IC package.
FIG. 5A shows an embodiment of the loop-forming tool 114, which may
conveniently be made of metal, ceramic, or reinforced plastic
material, or any combination of these materials. This loop-forming
tool 114, or its equivalent, is used to form a wire loop for the
present laser-driven microwelding or dual ball bonding system. The
loop-forming tool 114 can be made substantially smaller and thinner
in dimension than the conventional capillary bonding tool. As a
result, the loop-forming tool 114 is more compatible with the
miniaturized ball bond pitch enabled by the present invention.
The loop-forming tool 114 comprises a wire guide 152 and several
connected arms. One of the arms (i.e., arm 164) may be connected to
the aforementioned precision displacement subsystem through the
Z-axis linear precision displacement setup, so that the
loop-forming tool 114 can be vertically translated to desirable
levels during the ball bonding operation without affecting the
focus of the laser beams. In addition, to maximize its
maneuverability, the loop-forming tool 114 may have four degrees of
freedom of motion, i.e., X, Y, Z and .omega. (rotation).
The wire guide 152 captures and guides the wire during the
loop-forming process. It is designed to facilitate the formation of
a wire loop as well as to maintain the wire loop in a plane
essentially vertical to the substrate. In the particular embodiment
shown in FIG. 5A, the wire guide 152 takes the shape of a gable
roof, although it may take any of several shapes, e.g., a gambrel
roof or a cut-open pipe. The "height" 154 of the wire guide, or the
depth of the groove when viewed upside down, is preferably not less
than the size of the wire. To enhance the guiding capability of the
wire guide 152, its inner surface may be roughened or corrugated.
The angle of opening 156 of the wire guide 152 determines the range
of wire size that can be handled by the loop forming tool 114; a
typical value of 100.degree., together with roof edges of suitable
lengths, would enable the wire guide 152 to handle coarse wires,
e.g. 3-mil wires. The wire guide 152 forms an angle (the "loop
angle") 158 with a first, essentially vertical arm 160. The loop
angle 158 is critical in shaping the profile of the wire loop and,
in accordance with the specific bonding operation requirement, can
be either fixed or adjustable, e.g., from 30.degree. to 90.degree.,
via a pivot linking the wire guide 152 and the arm 160. Finally,
the length of the wire guide 152 provides a stable compressive
force and guiding effect to the wire and is preferably 10% to 100%
of the length of the first arm 160.
In the specific embodiment shown in FIG. 5A, the first arm 160 is
connected to a second, essentially horizontal arm 162. The lengths
of the arms 160 and 162 may be either fixed or adjustable on the
basis of specific wire sizes and bonding needs. A convenient way to
adjust the height of the wire loop is by adjusting the length of
the first arm 160. Similarly, the length of the wire loop may be
adjusted by the length of the horizontal arm 162. The second arm
162 is linked to a third arm 164, which in turn may be linked to
the precision displacement subsystem (not shown in FIG. 5A).
Also shown in FIG. 5A is a wire clamping tool 166, which may be
used to hold a wire 168 and to flatten the end of the wire 168 when
necessary. This wire clamping tool 166 may be connected to a wire
feeding mechanism, known to those skilled in the art, and should
preferably be positioned as close to the loop-forming tool 114 as
possible such that the wire loop length can be minimized.
FIG. 5B depicts another embodiment of the loop forming tool 114,
which may also be made of a variety of materials ranging from metal
to reinforced plastics. Again, the loop-forming tool 114 can be
made substantially smaller and thinner in dimension than the
conventional capillary bonding tool. As a consequence, this
loop-forming tool 114 is compatible with the miniaturized ball bond
pitch espoused by the present invention.
In this specific embodiment of the loop forming tool 114, a wire
guide 152 is directly connected to a motor-driven rotating shaft
172 via an arm 170. The wire guide 152 is used to capture and guide
the wire during the loop-forming process. The wire guide 152 of the
specific embodiment shown in FIG. 5B is essentially identical to
that illustrated in FIG. 5A. For example, it may takes the shape of
a gable roof or several other shapes, as discussed above; the inner
surface of the wire guide 152 may be roughened or corrugated to
enhance its guiding capability; and the angle 158 between the wire
guide 152 and the arm 170 may be either fixed or adjustable
depending on the specific bonding requirement. Finally, the arm 170
may have either a fixed or adjustable length depending on wire
sizes and other bonding operation requirements.
A wire clamping tool 166 is shown in FIG. 5B. It holds a wire 168
from a prior-art wire feeding mechanism, e.g., a reel or a wire
pool, 174. To illustrate the proximity of the loop forming tool
114, the wire clamping tool 166, and the bonding elements (e.g., a
bond pad 138 of an IC chip 140 and a bonding finger 176 of an IC
package 178) during operation, all of these are simultaneously
shown in FIG. 5B. Note that the IC chip 140 may be attached to the
IC package 178 via a die attach adhesive. Also shown in FIG. 5B is
a laser beam 130 generated by the aforesaid laser/focus/monitoring
subsystem, which beam 130 is focused upon a microwelding area
during operation.
FIG. 6 is a perspective view of a specific embodiment of the
laser-driven microwelding system of the present invention. The
coupled output beam 130 from both the pulsed source and the aiming
laser source (both contained in a source box 181) is transmitted
through an optical fiber 126 to the assembly 183 encompassing the
aforesaid beam imaging assembly, the beamsplitter compartment and
the CCD camera. As described above, this assembly 183 can be
translated vertically (i.e., in the Z-direction) with an accuracy
of 1 .mu.m by a linear precision translation stage known to those
skilled in the art. There are two optical outputs from this
assembly 183, one being the coupled laser beam while the other
being the split aiming beam transmitted to the monitor 136. Also
shown in FIG. 6 are: an X,Y precision translation system 182, in
which the X- and Y-direction motions may be independently
controlled; a rotating shaft 172 connected to a loop forming tool
114; a second motor-driven rotator 184 affording the loop forming
tool 114 another degree of rotational freedom; a wire clamping tool
166; a reel 174; and a substrate (e.g., IC chip) 140. Finally, the
perforated platform on which the substrate is located is connected
to a vacuum source (not shown) to hold substrate in place during
bonding operation.
Having described the several subsystems of the novel laser-driven
microwelding system, a discussion of the new dual ball bonding
process is in order. See FIGS. 7A-7J.
In the operation of the laser-driven microwelding apparatus of the
present invention, the wire clamping tool 166 first flattens the
tip of the wire 168 made of, e.g., copper, to increase the
cross-sectional area of the wire 168 for more effective laser
energy absorption; see FIG. 7A. Also shown in FIG. 7A are: a
substrate 140, a bond pad 138 on the substrate, a loop-forming tool
114, and an aiming laser beam 124.
In FIG. 7B, the flattened portion of the wire 168 is moved toward
the focal spot of the first laser beam 118a, resulting in the
formation of a Cu ball 190 at the tip of the wire as shown in FIG.
7C. Further advancing (threading) the wire slightly presses the Cu
ball 190 against the bond pad 138; see FIG. 7D. Also in FIG. 7D, a
second laser pulse 118b irradiates the Cu ball 190 to form a strong
alloy ball bond 192 between the Cu ball 190 and the aluminum bond
pad 138. This ball bond 192 is shown in FIG. 7E. Typically, an
approximately 2.times. to 3.times. ratio between the diameter of
the focused beam spot and that of the ball bond was observed.
In FIG. 7F, the loop-forming tool 114 is shown raised and
horizontally displaced, unreeling a small length of wire 168 while
capturing the wire within the wire guide 152. A wire loop 194 is
thus formed by the movement of the loop-forming tool 114 relative
to the substrate 140. The wire clamping tool 166 is then actuated
to flatten the wire 168 at a pre-selected location before it
releases this flattened section 196. In FIG. 7G, the loop forming
tool 114 holds the wire down so that the flattened portion 196
resides directly above, but does not contact, the bond pad 176 of
an IC package 178. This prevents the package bond pad 176 from
being damaged by the laser. A third laser pulse 118c then
irradiates the flattened portion of the wire to break the Cu wire
168 into two segments, with a new Cu ball formed at each broken end
202 and 204; see FIG. 7H.
In FIG. 7I, the loop forming tool 114 is again lowered, with the
wire held by the wire guide 152 also lowered, and thereby presses
the copper ball 202 against the package bond pad 178. Finally, a
fourth laser pulse 118d fuses the copper ball 202 of the wire loop
194 with the packaging bond pad 178. Thus, a closed Cu wire loop
194 is completed with high strength ball bonds at both the IC chip
bond pad 138 and the package bond pad 178; see FIG. 7J. The
loop-forming tool 114 is now ready to move on to the next bonding
position for the formation of the next wire loop.
To simplify laser design and operation, all the above four laser
pulses 118a, 118b, 118c and 118d may be of the same power density
and duration. That is, for the formation of dual copper-aluminum
ball bonds, a laser having a power density of approximately
4.7.times.10.sup.5 W/cm.sup.2 may be activated four times, each for
a duration of approximately 0.5 millisecond.
Having described the novel dual ball bonding process, attention is
turned to a new metal bumping process using the novel laser-driven
microwelding system. This new ball bumping process, as depicted in
FIGS. 8A-8G, may be carried out without the loop-forming tool shown
in FIG. 5A or 5B, although using it would accelerate the
auto-focusing process by pre-setting the loop forming tool at the
focal plane of the coupled laser beam.
Referring to FIG. 8A, the wire clamping tool 166 first flattens the
wire 168, increasing the wire cross-sectional area thereof to
intercept laser energy more efficiently. The aiming laser beam 124
may be used to locate the focal spot of the high-power laser beam.
The wire may be made of copper, gold, palladium, or a number of
other metals such as molybdenum or solder.
In FIG. 8B, the flattened portion 206 of the wire 168 is moved
toward the focal spot of the laser. A first laser pulse 118a
results in the formation of a Cu ball 208 at the tip of the wire
168; see FIG. 8C. Further threading the wire makes the Cu ball 208
touch the bond pad 138 with a light downward pressure. A second
laser pulse 118b irradiates the ball 208 to form a strong alloy
bond between the copper ball 208 and the aluminum bond pad 138; see
FIG. 8D.
In FIG. 8E, the Cu ball bonded to the bond pad 138 by the second
laser pulse now takes the shape of essentially a hemisphere 210.
Again, the presence of the loop-forming tool 114 facilitates and
accelerates the auto-focusing process. A small length of wire is
unreeled, before the wire clamp 166 is actuated again to break the
copper wire 168 right above the copper bump 210. Alternatively, a
third laser pulse 118c may irradiate the wire 168 right above the
bump 210, separating the wire 168 and the bump 210; see FIG. 8F.
This third laser pulse 118c also partially melts the top portion of
the bump 210 so that surface tension can restore the bump into an
essentially hemispherical shape; see FIG. 8G.
Note that the novel bumping process of the present invention does
not require the seed and barrier layers which are required by the
conventional bumping process. This bumping process also requires no
use of flux.
Having described the new ball bonding process and the new bumping
process, attention is now turned to a new TAB assembly process
using the new laser-driven microwelding system of the present
invention. This new TAB assembly process is depicted in FIG. 9.
In the present automated TAB assembly process, Cu ball are
preferably formed at the tip of all the Cu leads within the same
run before these Cu ball are fused with the corresponding bond pads
of the IC chip. Refer to FIG. 9A. A laser pulse 118a focuses
directly upon the tip of a Cu lead 220 on a TAB tape 40 and melts
it to form a Cu ball 222. The Cu ball 222 is later brought into
contact with a bond pad 138 of an IC chip 140 with a slight
compression. As known to those skilled in the art, IC chip 140 may
be attached to a sticky or wax layer 50 on a tape carrier 52; see
FIG. 9B. The IC chip and its bond pads are aligned with the Cu
leads of the TAB tape using optical alignment known to one skilled
in the art (thus not shown). A second laser pulse 118b then
irradiates the Cu ball 222 to melt the Cu ball again and form an
alloy with the matched bond pad 138.
In FIG. 9C, the TAB tape 40 is gently deflected toward the IC chip
surface by a hollow gang deflector 224 so that each of the copper
balls 222 is slightly compressed against its corresponding bond pad
138. This gang deflector 224 provides a slight physical
displacement to the TAB tape to ensure a slightly compressive
contact between the Cu ball and the bond pad. The gang deflector
224 can take various shapes, e.g., the shape of a square hollow
cylinder as shown in FIG. 9C. The contact surface between the gang
deflector 224 and the TAB tape 40 can also take various shapes,
e.g., a flat surface or a four-point contact at the corners of a
square hollow cylinder gang deflector.
A short laser pulse 118b subsequently irradiates and melts a Cu
ball 222. The molten Cu ball 222 amalgamates with the corresponding
bond pad 138 during solidification, forming an alloy metal bond.
The strength of these solder-free alloy bonds are much stronger
than those formed by conventional TAB assembly methods. Notably,
the novel TAB assembly process of the present invention does not
require any photolithography process or exposure to plating
chemicals, or the use of flux. Nor does the present TAB process
require any of the thin film deposition steps dictated by the
conventional TAB technique.
Although the present invention has been described above in terms of
several specific embodiments, it is anticipated that alterations
and modifications thereof will no doubt become apparent to those
skilled in the art having read the above detailed description of
the embodiments. It is therefore intended that the following claims
be interpreted as covering all such alterations and modifications
as fall within the true spirit and scope of the invention.
* * * * *